This invention relates to a magnetically shunted current transformer and, more particularly, to such a transformer for use with fault current indicators of power line distribution systems.
Current sensing transformers for sensing faults in a power line are well known in the art and generally comprise a pair of generally U-shaped ferrite cores which are mounted about the conductor. The abutting core sections carry a secondary winding which generally takes the form of a bobbin having a bore therethrough for accepting the core and having a multiple-turn winding thereabout. The conductor itself forms the primary winding for the transformer and current flowing through the conductor induces a corresponding current in the secondary or sensing winding. The secondary winding is provided in a "lumped" or non-distributed fashion so as to facilitate installation and removal of the transformer from energized power cables.
However, it has been found that these prior art current transformers saturate at relatively low current levels, are position sensitive and operate in a non-linear manner due to core saturation. These phenomena are due to the fact that leakage flux occurs at the portion of the core which is opposite from the secondary winding. Accordingly, the flux density in this portion of the core is substantially greater than the flux in other portions of the core as well as the transformer secondary itself. Thus, the current transformer core readily saturates and, therefore, the transformer operates non-linearily and is position sensitive to the conductor even though the magnetizing flux inside of the secondary winding is itself well below the saturation level of the core.
These and other disadvantages are overcome by the present invention wherein there is provided a magnetically shunted transformer which functions to recapture this leakage flux thereby allowing substantially higher conductor current levels to flow before transformer saturation occurs. The operation of the transformer in accordance with the present invention approaches that of an ideal current transformer and, accordingly, greater accuracy, range, sensitivity and reliability are achieved.